As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional optical amplifier comprises a gain medium, such as a single mode glass fiber having a core doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source. Pump energy that is within the absorption band of the rare earth dopant is combined with the optical input signal within the coupler, and applied to the medium. The pump energy is absorbed by the gain medium, and the input signal is amplified by stimulated emission from the gain medium.
Such amplifiers are typically used in a variety of applications including, but not limited to, amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed. Thus, a signal propagating on a silica-based optical fiber can be introduced to a silica-based optical fiber amplifier, and amplified by coupling pump energy into the amplifier gain medium.
Fiber amplifiers are generally constructed by adding impurities to (i.e. "doping") an optical fiber. For a silica-based fiber, such dopants include the elements erbium and ytterbium. For example, one type of fiber amplifier referred to as an erbium (Er) amplifier typically includes a silica fiber having a single-mode core doped with erbium ions (conventionally denoted as Er.sup.3+). It is well known that an erbium optical fiber amplifier operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1550 nanometers (nm). Likewise, an amplifier having a silica-based fiber "co-doped" with erbium and ytterbium shows excellent amplification of a 1550 nm optical signal when pumped with a wavelength from about 980 nm to about 1100 nm. A particularly useful pump wavelength is 1060 nm because of the availability of high power solid state laser sources at about 1060 nm. Since 1550 nm is approximately the lowest loss wavelength of conventional single-mode glass fibers, these amplifiers are well-suited for inclusion in fiber systems that propagate optical signals in the wavelength vicinity of 1550 nm.
It has been an ongoing pursuit in the field of optical fiber amplifiers to increase the power output of the amplifiers. Higher power levels permit transmission over longer distances before further amplification is necessary. In addition, newer wavelength division multiplexed systems have a relatively low power-per-channel, given a constant total output power. This increases the need for higher total amplifier output power. Traditionally, pump energy is applied to the gain medium by coupling it into the doped fiber either in the same propagation direction as the signal to be amplified (referred to as "co-pumping"), or by coupling it into the doped fiber in the opposite direction as the signal to be amplified (referred to as "counter-pumping). Each of these pumping methods has its own advantages, but also its own limitations. It is an object of this invention to go beyond these traditional pumping methods to provide a high power optical amplifier that uses a new means of optical pumping.